A FIMS (Field Induced Magnetic Switching) MRAM cell comprises a MTJ (Magnetic Tunnel Junction) in the form of a stacked memory element, a bit line (BL) metal running on the MTJ in a first direction, a write word line (WWL) running under the MTJ in a second direction crossing the first direction, a read word line (RWL) as a select gate running in the second direction and a read metal wire connecting the MTJ to a selected device. The MTJ is connected to the BL. An insulation layer is disposed between the MTJ and the WWL to prevent current flow through the MTJ during a write operation.
The MTJ is a stack comprising a pinned layer with fixed magnetism, a tunnel layer and a free layer. Two orthogonal currents carried by the write word line and the bit line manipulate magnetic fields in the free layer. When the direction of the free layer field coincides with that of the pinned layer, tunnel resistance of the MTJ becomes low. When the direction is opposite, the resistance becomes high. A memory high or low state is defined as high or low resistance of the MTJ element. A read word line (RWL) selects a column to be read.
There are two word lines, RWL and WWL, in a unit cell. The RWL or WWL play no role during “write” or “read” operations. Thus, having a RWL and a WWL is not only inefficient but also limits the miniaturization of a memory cell. WWL is patterned in the second or upper metals. A metal stud to connect the read metal is also formed at the WWL metal level. There are two sets of metal and space along bit line direction, and one set along the word line direction. Since the MTJ should be within widths of BL and WWL, overlay margin is added to widths of BL and WWL. Minimum feature size of 2nd or upper metal width is 1.5 to 2 times larger than gate width. The unit cell area becomes 30 to 40 F2, where F is minimum feature size of its technology.
Another serious issue with the conventional MRAM cell is that the write current to switch a vector of free layer is not scaled down with miniaturization. Thus, the problem of electro migration arises from high current density,